1. Field of the Invention
This invention relates to a pulse width modulated sine and cosine wave generator.
2. Description of the Prior Art
In the field of numerical control, resolver and Inductosyn feedback devices requires two sine waves ninety degrees apart or in quadrature. These sine waves can of course be generated with elaborate equipment, but the simplest way to maintain the precise quadrature relationship is by the use of waves derived from a counter driven by a constant frequency clock. However, as is well known, square waves are rich in harmonic content so that considerable filtering is required. It is therefore recognized in the art that it would be helpful to eliminate or suppress the undesired odd harmonics, particularly the third, fifth and seventh, to reduce filtering requirements.
An article appearing in EDN published by Cahners Publishing Co. Inc., Boston, Mass. 02116 on Jan. 15, 1971 entitled "Reduce Static Inverter Weight and Cost by Harmonic Neutralization" -- P. W. Koetsch, describes a technique for the suppression of the odd harmonics in a square wave output, utilizing a pulse width modulation technique. In this article there is described a static inverter, operating in the switching mode, that generates a square wave output that contains odd harmonics. Since the fundamental only was of interest, the pulse width technique was utilized to eliminate troublesome harmonics. In this technique, the on and off switching is controlled to produce a square pulse output wave of variable time width by using a stable oscillator and a countdown procedure.
The instant invention proposes to use the technique of pulse width modulation in a novel manner, to generate two sine waves exactly in quadrature, with minimal circuitry and greater flexibility in the selection of the pulse width pattern.